No. 592] SHORTER ARTICLES AND DISCUSSION 253 



hypothesis that there is to be ' ' no mixing of germ-plasms. ' ' But 

 what justification have we for that further hypothesis? Experi- 

 ence furnishes none. On the contrary, I have shown in numer- 

 ous specific cases that when unlike gametes are brought together 

 in a zygote, they mutually influence each other; they partially 

 blend, so that after their separation they are less different from 

 each other than they were before. The pure-line advocates have 

 adopted the procedure of dismissing such explanations as mysti- 

 cal, an easy way to dispose of troublesome ideas. But the stub- 

 born fact remains to be accounted for that partial blending does 

 occur (1) when polydactyl guinea-pigs are crossed with normals 

 (Castle, 1906), (2) when long-haired guinea-pigs are crossed 

 with short-haired ones (Castle and Forbes, 1906) and, (3) when 

 spotted guinea-pigs or rats are crossed with those not spotted 

 ( MacCurdy and Castle, 1907 ) . Davenport has furnished numer- 

 ous instances of the same thing in poultry ;. indeed he has shown 

 that "imperfection of dominance" and of segregation are the 

 rule rather than the exception in Mendelian crosses in poultry. 

 To assume that "there is no mixing of germ-plasms" is a con- 

 trary-to-fact assumption, whatever it may be in formal logic or 

 scientific methodology. 



Let us change slightly Pearl's hypothetical case. Let us sup- 

 pose, as he does, that a gamete a 38 has united in fertilization 

 with another a 38 gamete producing a soma, A 38 . Now what sort 

 of gametes may we expect such an individual to produce ? Pearl 

 says, in effect, nothing but a 38 gametes, unless a genetic miracle 

 occurs, a mutation, incapable of casual explanation. But we 

 should hesitate to characterize as miraculous anything which oc- 

 curs with regularity, and experience shows that this is what hap- 

 pens quite commonly, if not regularly, in such cases. The A 38 in- 

 dividual produces gametes a majority of which have the value a 38 , 

 but a few of which have a higher value, a 39 , and a few a lower 

 value, a 37 . For the correlation in value between soma and 

 gamete is not absolute. It is in many cases close, but not in- 

 variable, as I think Dr. Pearl would admit. If it be granted for 

 the sake of argument that gametic variation occurs, it is obvious 

 that we have grounds for expecting somatic variation in the fol- 

 lowing generation. For an a 39 gamete uniting with another 

 gamete like itself may be expected to produce a zygote of value 

 A 39 . Pearl maintains that such an event is without ' ' causation . ' ' 

 is incapable of prediction and control, that all we can do is to 



